First download ZoiPer from their website.
Once installed account setup is fairly straight forward, use the account details provided to you and follow the images below:
Fist step is to go to settings and create a new account.
Then select SIP account type (should be default).
Enter in your account details, user field is your extension number, password is your SIP password and domain is your sip address, if your sip port number is not 5060 then you will also need to add the port number in the format of sip.voipline.net.au:5061 for example.
You can then name the account, using the default name generated is best.
Your account has now been added and you should be ready to make and receive calls.